Layered double hydroxides (hereinafter referred to as “LDHs”) are mixed hydroxides of divalent and tri-valent metals having an excess of positive charge that is balanced by interlayer anions. They can be represented by the general formula (1):MII1-xMIIIx(OH)2An-x/n.yH2O  (1)
where MII and MIII are di- and tri-valent metal ions respectively and An- is the interlayer anion of valance n. The x value represents the proportion of trivalent metal to the total amount of metal ion present and y denotes variable amounts of interlayer water.
Common forms of LDH comprise Mg2+ and Al3+ (known as hydrotalcites) and Mg2+ and Fe3+ (known as pyroaurites), but LDHs containing other cations including Ni, Zn, Mn, Ca, Cr, and La are known. The amount of surface positive charge generated is dependent upon the mole ratio of the metal ions in the lattice structure, and the conditions of preparation as they affect crystal formation.
LDH compounds are of interest because they are considered to be useful as catalysts, catalyst precursors, catalyst supports, absorbents, anion exchangers, PVC stabilisers, flame retardants, medicinal antacids and as material for use in nanocomposites.
LDH particles are typically plate like in morphology. During preparation of LDH compounds, the plate like particles tend to aggregate together to form larger particles, typically having particle sizes in the range of microns or above. It has been found to be difficult to disperse the aggregated particles because of the strong interactions between the platy LDH nanosheets, such as electrostatic attraction via the common surface anions and hydrogen-bonds via water molecules.
LDHs can be prepared by forming a mixed solution containing the M2+ and M3+ ions in solution and adjusting the pH of the solution to an alkaline pH. This results in the coprecipitation of the LDH as solid particles. Other synthetic pathways to form LDHs, particularly those containing magnesium, include synthesis from Mg(OH)2 (brucite) and MgO (calcined magnesia) via incorporating trivalent metal ions, such as Al3+, and including anions. A number of other methods for producing LDHs have also been described.
Liu et al, “Liquid-Crystalline Phases of Colloidal Dispersions of Layered Double Hydroxides”, Chem. Mater. 2003, 15, 3240-3241, described the synthesis of colloidal Mg/Al LDH that was carried out using a non-steady coprecipitation method. The pH of an aqueous solution of mixed magnesium and aluminium chlorides was raised to 9.5 by adding 3.5 M NH3.H2O under vigorous stirring. The resulting precipitate was aged at room temperature for one hour. After filtration, the filter cake was washed thoroughly with deionised water. It was then collected and closed in a glass bottle for peptization in a thermostat at 80° C. for 24 hours. Well dispersed colloidal LDH particles were obtained. Transmission electron microscopy showed that most particles were roughly monodispersed platelets, hexagonal in shape, with diameters between 50 and 80 nm, and the electron diffraction pattern showed Mg/Al LDH particles were well-crystallined with hexagonal symmetry. The particle thickness of about 5 nm was also revealed. The Zeta potential of the particles was measured to be +39 millivolts. Dispersions of the LDHs in water were shown to form liquid crystalline phases.
J.-M. Oh et al, “The Effect of Synthetic Conditions on Tailoring the Size of Hydrotalcite Particles”, Solid State Ionics, 151 (2002), 285-291, investigated preparation of hydrotalcites. In particular, clear metal solutions containing magnesium and aluminium were titrated up to pH of approximately 11 with sodium hydroxide solution containing sodium carbonate and aged in an autoclave at 100° C. for 12, 24, 48, 72 hours, and also at 100° C., 125° C., 150° C., 180° C., for 48 hours respectively. The particle sizes were analysed and it was found that increasing aging time and increasing temperature result in increasing particle size. The average particle size ranged from 85 nm (for aging at 100° C. for 12 hours) to 340 nm (for aging at 180° C. for 48 hours).
European patent application no. 987328 in the name of Jin Ho Choy described a bio-inorganic hybrid composite for retaining and carrying bio-materials with stability and reversible dissociativity. The bio-inorganic hybrid composite was prepared by forming a stable layered double hydroxide in which anions are intercalated and subjecting the intercalated anions to an ion exchange reaction with a bio material. The bio material is suitably nucleoside-5′monophosphate, nucleoside-5′triphosphate, or a gene material with a size of 500-1000 base pairs.
The applicant does not concede that the prior art discussed above forms part of the common general knowledge in Australia or elsewhere.
Throughout this specification, the word “comprising” or its grammatical equivalents shall be taken to have an inclusive meaning unless the context clearly indicates otherwise.